This invention relates to feedback control arrangements for lasers in fiber optic lightwave systems.
For injection lasers to be used as practical signal sources for lightwave communications, the average optical power, as coupled into a transmission fiber, and the peak optical power in the case of digital systems, or the depth of modulation in the case of analog systems, should remain essentially constant as the ambient temperature changes and as the lasers age. If it is assumed that the slope efficiency of the light-current characteristic of the lasers does not change with time or temperature and, in the case of digital systems, that a pseudo-random signal is always present at the laser terminals, then the peak power or the depth of modulation will be constant if the average power is kept constant. One circuit developed for this purpose stabilizes the average output power of the laser by using a p-i-n photodiode to derive a feedback control signal from the back mirror emission of the laser, while the output, coupled directly into the end of a fiber, is taken from the other, or front mirror. This back-mirror monitoring scheme is described by P. W. Shumate et al in BSTJ, 57, 1823 (1978).
The causes of instability in fiber output power associated with the back-mirror monitoring scheme were pointed out by F. Chen et al in Applied Optics, 17, 2219 (1978). They are: (1) the change of the laser-fiber coupling due to mechanical displacement and/or mode instability of the laser and (2) change in the ratio of powers from the front and back mirrors as the lasers age (front/back mirror mistracking). In order to ensure that the fiber output remains constant, it is necessary to monitor the power in the fiber and use that signal for feedback control.
To monitor the fiber power, an optical tap has been developed by M. DiDomenico et al, U.S. Pat. No. 4,165,496, issued on Aug. 21, 1979. In this tap, a beam splitter is formed by positioning beveled, parallel end faces of two segments of the transmission fiber in coaxial alignment and in close proximity to one another. A photodiode in a feedback circuit detects the portion of light reflected out of the fibers by the beam splitter. Transmitter packages using these taps for feedback control have been built. The burn-in test of these packages indeed showed a significant improvement in the stability of fiber output compared with the back-mirror monitoring scheme. However, since the ratio of fiber output to the tap output is very sensitive to a small laser-fiber misalignment due to the tap's mode selectivity, further improvements in the stability of the fiber output, especially regarding temperature variation, require mode mixers.
A variation of the tap-monitoring scheme is the use of a partially reflecting mirror to tap off part of the laser power from its front-mirror for feedback control. However, because the performance of the transmitter packages depends critically on the optical alignment of lasers, fibers, lenses, and mirrors, and on their mechanical stability in a field environment, any additional optical components may not only increase cost but even possibly degrade reliability of the packages.
In each of the above schemes, laser light is coupled axially into the end of a transmission fiber either directly or through a discrete lens or through a lens formed by shaping the fiber end. However, in U.S. Pat. No. 4,092,061, D. J. Stigliani, Jr., teaches a coupler in which a fiber with a beveled end face is oriented transversely to the laser axis so that the laser beam, which is focused through the cylindrical surface of the fiber, is reflected by the beveled face into the fiber core. Thus, the fiber itself acts as a cylindrical lens for light emitted in the plane normal to the laser junction where the beam divergence is large and, therefore, focusing is needed most for efficient coupling into the fiber; and the beveled end face of the fiber (polished, say, at 45 degrees to the fiber axis) acts as a mirror to reflect the light into the fiber. However, the power coupled into the fiber using this scheme is as sensitive to changes in laser-fiber coupling induced by mechanical motion as is the back-mirror monitoring scheme and the tap-monitoring scheme where no mode mixers are used.
Although a well-designed laser package should allow no relative mechanical motion between the laser and the output fiber, nevertheless these motions do occur.